Remove database module (not used)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Raise NotImplementedError when needed

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Drop expired stored stanzas silently (close #43)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>